The objective of this research is to examine two related questions: (1)\the relation between measured intelligence and family size; and (2) the relation between measured intelligence and birth order. The analyses will be based on data from the Wisconsin Longitudinal Study. The analysis of the relation between intelligence and family size will focus on estimating the generational change in mean IQ in two ways, first between parental and respondent generations using data on sibship size, and second between respondent and offspring generations using data from reproductive histories of men and women in the Wisconsin sample. Our analysis will emphasize the second of these two approaches, which will be complemented by path analyses of social background effects on IQ, education, age at marriage, and fertility. Questions of interpretation, centering on the relative importance of genetic and environmental factors on measured IQ, will be addressed. The research on the influence of birth order on IQ will first be done in the usual way using data for the full Wisconsin sample. But the second and more novel approach will use the respondent-sib difference in IQ, indexed by birth orders of respondent and sib, as the dependent variable. This within-family difference automatically controls for many confounding factors, since respondent and sib share the same parents. The second approach, therefore, promises to give more informative results than the first. Statistical controls will be introduced via multiple regression and multiple classification analysis. The research will address questions about population quality deriving from the associations of size of family of origin and sibling position with measured intelligence and the association of measured intelligence with fertility.